In recent years, in response to introduction of carbon dioxide emission control for environmental protection, heat ray-shielding glasses that are capable of suppressing the inflow of heat from outside, particularly sunlight, during summertime have been drawing attention as window glasses of vehicles such as automobiles and trains and those of buildings.
As one example of such heat ray-shielding glasses, one in which a heat ray-absorbing material is incorporated into a glass or an interlayer used in a laminated glass is known (for example, Japanese Patent Application Laid-Open Publication (JP-A) No. 2010-17854). However, since such a heat ray-absorbing material converts the sunlight incoming from outside into thermal energy, there is a problem that the thus generated heat radiates into the room and the heat ray-shielding efficiency is consequently reduced. In addition, absorption of heat radiation partially increases the glass temperature and the resulting difference between the glass temperature and the ambient temperature may cause damage to the glass itself.
Meanwhile, there are also known heat ray-reflecting glasses in which a heat ray-reflecting film is formed on a glass or a film having a heat ray-reflecting function is inserted into a laminated glass. In these heat ray-reflecting glasses, incoming lights including infrared radiation are reflected to the outside. Therefore, lights and heat do not flow into the room and heat rays can thus be more effectively blocked. In addition, since an increase in the glass temperature caused by heat rays can be suppressed, damage to the glasses can be inhibited as well.
Representative examples of a method for obtaining such a heat ray-reflecting glass include a method in which a metal film is formed on a glass surface by sputtering or the like (for example, Japanese Patent No. 3901911). However, although a metal film reflects heat rays, it also unevenly reflects visible light and the reflection intensity varies depending on the wavelength; therefore, there is a problem that the resulting glass shows coloration. In addition, since a metal film cannot selectively reflect a light of visible wavelength region and a light of near-infrared region, it is difficult to improve the heat ray-shielding performance of a glass while maintaining its visible light transmittance. Moreover, since a metal film has a radio wave-blocking property, it may make an instrument such as a cellular phone unusable.
Furthermore, as other example of heat ray-reflecting glass, laminated glasses into which a polymer multilayer film prepared by alternately laminating polymers having different refractive indices is inserted are known (for example, Japanese Patent No. 4310312). In such a polymer multilayer film, the wavelengths to be reflected can be selectively chosen by controlling the layer thickness; therefore, the film can be made to selectively reflect the lights of near-infrared range, so that the heat ray-shielding performance of a glass can be improved while maintaining its visible light transmittance. In addition, since such a polymer multilayer film does not contain any radio wave-blocking material such as a metal, excellent radio wave permeability can be retained. However, in cases where such a multilayer laminate film is used, as the light incident angle with respect to the film surface increases, the wavelengths of lights that can be reflected shift toward the lower wavelength side and this results in a change in the color tone. Accordingly, to obtain a heat ray-reflecting glass showing no color change, since the light reflection bandwidth viewed from the front side is required to be arranged in the near-infrared region which is further away from the visible wavelength region, there is a problem that the heat ray-shielding performance cannot be improved. Furthermore, in the near-infrared region where such a multilayer laminate film can mainly reflect lights, only a small fraction of sunlight can be reflected; therefore, such heat ray-shielding performance cannot necessarily be considered sufficient.
It could therefore be helpful to provide a multilayer film having superior heat-ray shielding performance as compared to conventional polymer multilayer films, in which multilayer film the changes in color caused by different viewing angles are suppressed.